This project involves utilization of single neuron recording and operant conditioning techniques in behaving monkeys to study brain mechanisms underlying voluntary movement. Monkeys are trained to make precise movements of a handle whose movement controls a visual display, and stimuli are delivered via the handle by means of an electronically controlled torque motor in order to determine how sensory feedback is processed. Using these methods we have shown that (1) motor cortex pyramidal tract neurons (PTNs) exhibit intense modulation during precise small movements involving relatively slight changes in amount of muscle activity. (2) A large proportion of the PTNs in primary motor cortex (MI) are engaged in controlling early-recruited motoneurons during finely-graded movements. (3) MI PTNs are strongly modulated with different loads especially in the vicinity of zero load. (4) There are significant differences between large and small PTNs with respect to their load-frequency relations.